1. Field of the Invention
The present invention is in the field of semiconductor manufacturing, and it specifically relates to a grinding process for removing the waviness of the surface of a freshly sawed silicon wafer.
2. The Prior Art
In a current practice, boules of single-crystal silicon range from 100 to 300 millimeters in diameter and are cylindrical in shape. These boules are cut with a wire saw or with an internal diameter saw into disc-shaped wafers approximately one millimeter thick. The wire saw reduces the kerf loss and permits many wafers to be cut simultaneously. However, the use of these saws results in undesirable waviness of the surfaces of the wafer. The surfaces need to be made more plane (planarized) before they can be polished, coated, or subjected to other processes. It is not unusual for the amplitude of the waves in each surface of a wafer to exceed 30 micrometers, and application of the process of the present invention typically renders each surface flat to within a fraction of one micrometer.
In one prior art method, which might be characterized as a grinding method, a first surface of the wafer is drawn or pushed against a hard flat holder, thereby elastically deforming the wafer, while the second surface of the wafer is ground flat. When the wafer is released, elastic restoring forces in the wafer cause it to resume its original shape, and it can be seen that the waves in the first surface have been transferred to the surface that has been ground. Thus while this technique produces a wafer of more uniform thickness, it does not eliminate the residual saw waves.
In a second prior art method, which might be called a lapping method, the wafer is simultaneously lapped on both sides with an abrasive slurry in a lapping machine. Compared to a grinding process, the lapping process is slow and must be followed by careful cleaning and an etching step to relieve stresses before the wafer is polished. These additional steps cause the conventional method to be more expensive and time-consuming than the method of the present invention. Also, the etching process employed after the lapping step is undesirable from the environmental standpoint, because the strong acids used must be disposed of in an acceptable way. The present invention eliminates the need for the etching step.
The first of the above described prior art methods is illustrated by U.S. Pat. No. 4,918,869 to Kitta. This patent shows a technique in which the wafer is bonded to a rigid pressing plate. The rigid pressing plate is pushed by a diaphragm to press the wafer against a turntable.
In U.S. Pat. No. 5,205,082 issued Apr. 24, 1990, Shendon, et al. show a polishing apparatus in which an insert is used to adhere the wafer to the surface of a carrier. A flexible but impermeable diaphragm connects the carrier to the remainder of the polishing head.
In U.S. Pat. No. 5,212,910 issued May 25, 1993, Breibogel, et al. describe a composite polishing head having a 3-layer structure that enables the pad to conform to longitudinal gradations across the wafer.
In U.S. Pat. No. 5,230,184 issued Jul. 27, 1993, Bukhman describes a polishing head that has a flexible membrane. A number of polishing pads, made from a wafer that has been sawed into small pieces are attached to the exposed surface of the flexible membrane. A controlled air pressure operating against the flexible membrane forces the polishing pads against the workpiece with uniform force.
Each of the methods described above suffers from one or more limitations, such as high cost, slow speed, or lack of performance. Accordingly, the present inventors have searched for and found a novel method of exceptional performance and have devised apparatus for carrying out the method in an efficient manner.